Decarburizing molten steel

ABSTRACT

THE APPLICATION DESCRIBES A METHOD FOR DECARBURIZING MOLTEN STEEL. IT COMPRISES THE STEPS OF INTRODUCING OXYGEN INTO A VESSEL CONTAINING A LIQUID BATH OF METAL, IN A MANNER WHICH PRECIPITATES A REACTION BETWEEN CARBON WITHIN THE METAL AND OXYGEN, ANALYZING THE GASES EXITING FROM THE VESSEL, DETERMINING WHETHER THE LANCE IS SUBMERGED FROM THE ANALYSIS OF THE EXITING GASES, AND LOWERING THE LANCE RESPONSIVE TO THE GAS ANALYSIS SHOULD IT NOT BE SUBMERGED, SO THAT OXYGEN IS ONLY BEING INTRODUCED BY SUBMERGED MEANS.

Aug. 27, 1974 H. L. BISHOPJR 3,332,150

DECARBURI Z ING MQLTEN STEEL Filed Dec. 30. 1971 INVENTOR.

HARRY L. BISHOP, JR.

A! forney United States Patent 01 p566 Patented Aug. 27, 1974 3,832,160 DECARBURIZING MOLTEN STEEL Harry L. Bishop, Jr., Pittsburgh, Pa., assignor to Allegheny Ludlum Industries, Inc., Pittsburgh, Pa.

Continuation-impart of abandoned application Ser. No.

864,279, Sept. 30, 1969. This application Dec. 30, 1971,

Ser. No. 214,446

Int. Cl. C21c 5/32 US. C]. 75-60 22 Claims ABSTRACT OF THE DISCLOSURE The application describes a method for decarburizing molten steel. It comprises the steps of introducing oxygen into a vessel containing a liquid bath of metal, in a manner which precipitates a reaction between carbon within the metal and oxygen, analyzing the gases exiting from the vessel, determining whether the lance is submerged from the analysis of the exiting gases, and lowering the lance responsive to the gas analysis should it not be submerged, so that oxygen is only being introduced by submerged means.

This application is a continuation-in-part of copending application Ser. No. 864,279 filed Sept. 30, 1969, and now abandoned.

The present invention relates to a decarburizing process and more particularly to a decarburizing process which controls lance position by analyzing the gases exiting from the vessel.

Decarburizing is an essential part of present-day steelmaking processes. It is generally performed by introducing oxygen into a vessel; e.g., an electric furnace, an open hearth furnace, or a basic oxygen furnace (BOF), containing a liquid bath of metal, in a manner which precipitates a reaction between carbon within the metal and oxygen. The oxygen is blown through a lance which is positioned on top of or submerged below the liquid bath level. A lance is considered to be submerged when its oxygen outlet openings are below the liquid bath level; i.e., the level of the metal and metal emulsion within the vessel.

I have found submerged blowing to be superior to top blowing. Submerged blowing is accompanied by a more predictable and efficient utilization of oxygen since top blown oxygen reacts with carbon monoxide rising from the bath in preference to carbon with in the bath. It additionally reduces the incidence of equipment damage and decreases the degree of splashing. Splashing is hazardous to operating personnel and can result in the loss of metal and/or slag, together with its contained metallic values; e.g., chromium.

A shortcoming of submerged blowing is the difiiculty in determining lance position. The lance should be immersed below the liquid bath level so that the advantages of submerged blowing can be attained. On the other hand, it should not be excessively immersed. Excessive immersion could damage the lining of the vessel by bringing the lance too close to the bottom of the bath and would cause consumable lances to erode at too fast a rate. I have overcome the problem of insufiicient or excessive lance lowering by developing a submerged blowing decarburizing process which controls lance position through analysis of the gases exiting from the vessel.

It is accordingly an object of the invention to provide a novel decarburizing process.

It is a further object of this invention to provide a decarburizing process which controls the lance position by analyzing the gases exiting from the vessel.

The foregoing and other objects of the invention will be best understood from the following description, reference being had to the accompanying drawing wherein:

FIG. 1 is a schematic diagram illustrating how the rate of carbon removal varies with carbon content for a fixed oxygen input during both top and submerged blowing; and

FIG. 2 is a schematic diagram illustrating how the analysis of the exiting gases varies with carbon content for a fixed oxygen input during both top and sumberged blowing.

The process of this invention is applicable to both plain carbon and aloy steels; e.g., stainless steels. It comprises the steps of introducing oxygen into a stationary vessel (non-rotary) containing a liquid bath of metal, in a manner which precipitates a reaction between carbon within the metal and oxygen, analyzing the gases exiting from the vessel, determining whether the lance is submerged from the analysis of the exiting gases and lowering the lance responsive to the gas analysis, should it not be submerged. The determination as to whether the lance is submerged could be made by operating personnel or by automatic equipment. The automatic equipment could comprise a gas analyzer which would transmit a signal to the lance position control regulator which would in turn, lower the lance at the appropriate time. Lance lowering is generally discontinued responsive to a change in the gas analysis which indicates submersion of the lance. Lowering of consumable lances which erode and egress to a level above the submersion level is often accomplished by lowering the lance a predetermined distance.

The pressure in the vessel could be below, at, or above atmospheric pressure. Alloy steels are often decarburized at a pressure below atmospheric and/ or in a vessel into which inert gas is introduced. Well known equilibrium relationships point out that it is essential at atmospheric pressure to reach very high melt temperatures at the end of the oxygen blow, in order to decarburize to low carbon levels and maintain relatively high chromium contents. These high temperatures are generally not attained. It is, therefore, desirable to lower the partial pressure of carbon monoxide in the vessel. A lowering of the partial pressure carbon monoxide changes the equilibrium relationships and shifts the attainable end point carbon to low levels at lower temperatures without requiring a reduction in the chromium content of the bath. Reduction in the partial pressure can be accomplished by reducing the pressure in the vessel and/or by introducng inert gas; e.g., argon.

The analysis of exiting gases is suitable for a determination as to whether the lance is submerged since exiting gases produced during top blowing differ from exiting gases produced during submerged blowing. For example, exiting gases resulting from top blowing contain a higher percentage of carbon dioxide than do exiting gases resulting from submerged blowing. As pointed out above, top blown oxygen reacts with carbon monoxide rising from the bath, thereby forming carbon dioxide, in prefrence to reacting with carbon within the bath, thereby forming carbon monoxide. Other differences in the analysis of exiting gases obtained from top blowing and submerged blowing include variations in carbon monoxide content, temperature and oxygen content. Gases resulting from top blowing have lower carbon monoxide contents, higher temperatures and sometimes higher oxygen contents than do gases resulting from submerged blowing.

Although the analyses for exiting gases obtained from top and submerged blowing cannot be precisely predetermined and calculated, they can be determined empirically. This is because the analyses are dependent upon many variables peculiar to the particulars of the decarburizing process being performed. These particulars include the composition of the gaseous blowing stream; e.g., diluent gas might be introduced into the vessel with the oxygen,

thetemperature of the liquid bath and the chemistry of the liquid bath.

FIG. 1 schematically shows how the rate of carbon removal varies with carbon content for a fixed oxygen input during both top and submerged blowing. The solid line reflects the rate for submerged blowing and the dotted line reflects the rate for top blowing. It is apparent from the diagram that carbon is removed at a faster rate during submerged blowing than during top blowing. This is because top blown oxygen reacts with carbon monoxide exiting from the vessel in preference to carbon within the vessel.

FIG. 2 schematically shows how the percentage of car bon monoxide and carbon dioxide in the exiting gases varies with carbon content for a fixed oxygen input during both top and submerged blowing. Solid lines A and B respectively represent the change in carbon monoxide and carbon dioxide during submerged blowing. The gradual decrease in carbon monoxide and increase in carbon dioxide is attributed to the reduced tendency for carbon and oxygen to react as the carbon content in the bath decreases. The dotted lines A and B respectively indicate how the analyses of carbon monoxide and carbon dioxide changes if the lance egresses to a level above the submersion level. Note the sharpness and distinctness in the change of the gas analysis.

It will be apparent to those skilled in the art that the novel principles of the invention disclosed herein in connection with specific examples thereof, will suggest various other modifications and applications of the same. It is accordingly desired that in construing the breadth of the appended claims they shall not be limited to the specific examples of the invention described herein.

I claim:

1. In the method of decarburizing molten steel wherein oxygen is introduced into a vessel containing a liquid bath of metal to react with carbon contained within the metal, an improved means of positioning a lance to insure that it is submerged within a liquid bath and yet not excessively submerged to the point at which it will cause excessive damage to the vessel, which comprises introducing oxygen into the vessel through a lance positioned above the liquid bath, analyzing the gas exiting from the vessel for at least one characteristic from the group consisting of its carbon dioxide content, carbon monoxide content, carbon dioxide and oxygen content, and temperature, lowering the lance while continuing to introduce oxygen, simultaneously with said lowering detecting a sharp and distinct change in the gas analysis which indicates that the lance is submerged within the liquid bath and that oxygen is only being introduced into said vessel by means located below the liquid bath level, and discontinuing said lowering, said change being from the group consisting of a decrease in the carbon dioxide content, an increase in the carbon monoxide content, a decrease in the carbon dioxide and oxygen content, and a decrease in the temperature.

2. A method according to claim 1 wherein said analyzing comprises the step of measuring the percentage of carbon dioxide in the exiting gas and wherein said change is a decrease in said percentage of carbon dioxide.

3. A method according to claim 1 wherein said analyzing comprises the step of measuring the percentage of carbon monoxide in the exiting gas and wherein said change is an increase in said percentage of carbon monoxide.

4. A method according to claim 1 wherein said analyzing comprises the step of measuring the percentage of carbon dioxide and oxygen in the exiting gas and wherein said change is a decrease in said percentage of carbon dioxide and oxygen.

5. A method according to claim 1 wherein said analyzing comprises the step of measuring the temperature of the exiting ga and wherein said change is a decrease in said temperature.

6. A method according to claim 1 adapted to produce alloy steel and including the step of reducing the partial pressure of carbon monoxide in said vessel.

7. A method according to claim 6 wherein said partial pressure of carbon monoxide is reduced by reducing the pressure in said vessel.

8. A method according to claim 6 wherein said partial pressure of carbon monoxide is reduced by introducing inert gas into said vessel.

9. A method according to claim 8 wherein said inert gas is argon.

10. In the method of decarburizing molten steel wherein oxygen is introduced into a vessel containing a liquid bath of metal to react with carbon contained within the metal, an improved means of positioning a consumable lance to insure that it is submerged within a liquid bath and yet not excessively submerged to the point at which it will cause excessive damage to the vessel and erode too rapidly, which comprises introducing oxygen into the vessel only through means located below the liquid bath level, said means below the liquid bath level being comprised of a submerged consumable lance, analyzing the gas exiting from the vessel for at least one characteristic from the group consisting of its carbon dioxide content, carbon monoxide content, carbon dioxide and oxygen content, and temperature, detecting a sharp and distinct change in the gas analysis which indicates that the consumable lance has eroded to a level above the level of submersion, and lowering and resubmerging the consumable lance responsive to said change in the gas analysis, so that oxygen is once again only being introduced into the vessel by means located below the liquid bath level, said change being from the group consisting of an increase in the carbon dioxide content, a decrease in the carbon monoxide content, an increase in the carbon dioxide and oxygen content, and an increase in the temperature.

11. A method according to claim 10 wherein said analyzing comprises the step of measuring the percentage of carbon dioxide in the exiting gas and wherein said change is an increase in said percentage of carbon dioxide.

12. A method according to claim 10 wherein said analyzing comprises the step of measuring the percentage of carbon monoxide in the exiting gas and wherein said change is a decrease in said percentage of carbon monoxide. I

13. A method according to claim 10 wherein said analyzing comprises the step of measuring the percentage of carbon dioxide and oxygen in the exiting gas and wherein said change i an increase in said percentage of carbon dioxide and oxygen. 7

14. A method according to claim 10 wherein said analyzing comprises the step of measuring the temperature of the exiting gas and wherein said change is an increase in said temperature.

15. A method according to claim 10 adapted to produce alloy steel and including the step of reducing the partial pressure of carbon monoxide in said vessel.

16. A method according to claim 15 wherein said partial pressure of carbon monoxide is reduced by reducing the pressure in said vessel.

17. A method according to claim 15 wherein said partial pressure of carbon monoxide is reduced by introducing inert gas into said vessel.

18. A method according to claim 17 wherein said inert gas is argon.

19. A method according to claim 10 wherein said lance is lowered a predetermined distance.

20. A method according to claim 10 wherein said lance lowering is discontinued responsive to a further change in said gas analysis which indicates submersion of said lance.

21. A method according to claim 1 wherein said vessel is a stationary vessel.

22. A method according to claim 10 wherein said vessel is a stationary vessel.

References Cited UNITED STATES PATENTS 6 3,252,790 5/1966 Kri-vsky 75-60 3,377,158 4/1968 Meyer 75-60 3,432,288 3/1969 :Ardito 75-60 L. DEWAYNE RU'ILEDGE, Primary Examiner P. D. ROSENBERG, Assistant Examiner US. Cl. X.R. 75-59 

